1. Field of the Invention
The present invention relates to a propeller shaft and a method for producing the same and, more particularly to a propeller shaft characterized by a joint construction between a propeller shaft main body and a yoke and a method for producing the same.
2. Description of the Related Art
A propeller shaft is a constituent component of the power transmission system of a front-engine and rear-wheel drive automotive vehicle which functions to transmit power generated by the engine to the rear wheels. The propeller shaft is, for example, disposed between the transmission and the differential of the automotive vehicle to function to transmit the rotating torque of the engine to the rear wheels.
In general, the propeller shaft comprises a tubular shaft main body, a yoke joined to one end of the shaft main body for connection to the transmission and a yoke joined to the other end of the shaft main body for connection to the differential. In recent years, to meet a demand for light-weight propeller shafts, propeller shafts have tended to be used in which the shaft main body is formed of fiber reinforced plastic (hereinafter, referred to as xe2x80x9cFRPxe2x80x9d). In the propeller shafts having the shaft main body of FRP, the metallic yoke is joined to the shaft main body with a part thereof being fittingly inserted into the shaft main body. In order to transmit high rotating torque generated by the engine to the rear wheels without any loss therebetween, it is crucial to prevent the occurrence of slippage between an inner circumferential surface of the shaft main body and an outer circumferential surface of the part of the yoke which is fittingly inserted into the shaft main body at the joint portion of the shaft main body and the yoke so as to assure the proper transmission of the torque therebetween. To make this happen, in many propeller shafts, serrations having axially extending teeth are formed in the outer circumferential surface of the relevant part of the yoke, so that this serrated portion is fittingly inserted into the shaft main body to be joined thereto for transmission of sufficient torque (in this specification, the term xe2x80x9caxiallyxe2x80x9d means an axial direction of the propeller shaft, and hereinafter, unless otherwise stated, the term xe2x80x9caxiallyxe2x80x9d means the axial direction of the propeller shaft).
On the other hand, there has been a tendency for a crushable construction to be adopted on the body of the automotive vehicle so as to absorb impact energy generated at the time of collision of the vehicle with another vehicle by the body to alleviate the impact that would otherwise be imparted to the passengers with a view to securing the safety of the passengers at the time of such a collision. In the event that the automotive vehicle having the crushable construction encounters in an accident involving a frontal or rear-ended collision, that is to say, that a compression load is applied to the propeller shaft in an axial direction, it is considered that the propeller shaft acts just like an impact returning rod or a prop and interferes with the absorption of the impact energy by the body. To cope with this, with a view to eliminating the interference with the absorption of impact by the body, there have been developed propeller shafts which are designed based on a concept in which the yoke sinks into the shaft main body at the time of collision. In this type of propeller shafts, it is desirable that a force required when the yoke sinks into the shaft main body or the sinking force is as small as possible.
In the propeller shaft having the aforesaid construction, however, the serrations are formed in the outer circumferential surface of the yoke to assure the proper transmission of torque, and in the event that an axial compression load is applied to the propeller shaft, the yoke has to sink into the shaft main body while the teeth of the serrations cut in the inner circumferential surface of the shaft main body, this extremely increasing the sinking force.
To cope with this, studies were made to develop technologies for reducing the sinking force of the propeller shaft with the yoke having serrations formed in the outer circumferential surface thereof, and as an example of the technologies so developed, Japanese Unexamined patent Publication 7-208445 (JP-A-7-208445) discloses a propeller shaft in which a shaft main body comprises a main layer extending over the whole of the shaft body in an axial direction thereof and a partial layer provided inside the main layer and including reinforcing fibers, wherein a yoke has a wedge having a leading end which confronts an inter-layer portion between the main layer and the partial layer, whereby when an axial compression load is applied to the yoke, the wedge separates the main layer and the partial layer from each other and sinks into the shaft main body while tearing open the shaft main body.
In this propeller shaft, the yoke having serrations formed in the outer circumferential surface thereof is press fitted into the shaft main body so as to be joined to the inner circumferential surface of the partial layer, and the transmission of rotating torque is secured between the partial layer and the yoke. On the other hand, when an axial compression load is applied to the yoke, the partial layer and the main layer of the shaft main body are separated from each other, and the yoke sinks into the shaft main body together with the partial layer to thereby reduce the sinking force. Namely, according to the construction of this propeller shaft, the aforesaid problem is solved by a construction in which the propeller shaft is divided into a portion for securing high rotating torque transmission properties and a portion for securing a small sinking force.
However, while the propeller shaft disclosed in JP-7-208445 is said to provide a smaller sinking force when compared to a conventional propeller shaft in which a yoke having serrations formed in an outer circumferential surface thereof is simply fittingly inserted into a shaft main body (hereinafter, referred to as a xe2x80x9cconventional propeller shaftxe2x80x9d), the sinking force provided thereby still does not meet the satisfactory level as a certain force is still required to separate the integrally formed main and partial layers from each other. In addition, in this propeller shaft, even when there is applied no axial compression load to the yoke or during a normal operation, depending upon the condition where the main and partial layers are integrally formed, it may be considered that the two layers separate from each other. Furthermore, since the main and partial layers have to be provided on the shaft main body and the wedge has to be provided on the yoke, the production process becomes complicated, this increasing the production cost. Thus, the propeller shaft disclosed in JP-7-208445 is still suffering from the aforesaid problems at this point in time.
The present invention was made to solve the aforesaid problem of compatibility of sufficient torque transmission with a smaller sinking force, which propeller shafts are facing, which each have a mechanism in which a yoke sinks into a shaft main body thereof, and an object thereof is to provide a propeller shaft that can provide a smaller sinking force while securing good rotating torque transmission properties, and which is simple in construction and low in production cost, by forming in advance grooves in an inner circumferential surface of a shaft main body for serration teeth to travel therealong when a yoke sinks into the shaft main body. The other object of the invention is to provide a method for producing the same propeller shaft through a simple work.
With a view to attaining the objects, according to a first aspect of the invention, there is provided a propeller shaft comprising a tubular shaft main body made of FRP and a metallic yoke having an inserting portion which has serrations having axially extending teeth formed in an outer circumferential surface thereof and mounted in at least one end portion of the shaft main body with the inserting portion being slide fitted into an inner circumferential surface of the one end portion of the shaft main body, wherein the shaft main body comprises serration fit-in grooves formed on the inner circumferential surface thereof and having serration fit-in portions into which the teeth of the serrations are allowed to fit and extending portions formed continuously with the serration fit-in portions so as to extend more inwardly from axially central side ends of the teeth of the serrations toward an axial center of the shaft main body.
In other words, the propeller shaft according to the invention has formed in the inner circumferential surface thereof the serration fit-in grooves having the serration fit-in portions and the extending portions and thereby provides a mechanism in which during a normal operation, the teeth of the serrations formed in the outer circumferential surface of the inserting portion of the yoke come into mesh engagement with the serration fit-in portions of the serration fit-in grooves formed on the inner circumferential surface of the shaft main body to thereby transmit rotating torque properly, whereas, when an axial compression load of certain magnitude is applied to the yoke, the teeth of the serrations are allowed to travel along the extending portions formed continuously with the serration fit-in portions, whereby the yoke is allowed to sink into the shaft main body.
However, in conventional propeller shafts, no such extending portions of the serration fit-in grooves are formed on the inner circumferential surface of the shaft main body, and therefore, in order to allow the yoke to sink into the shaft main body, the axially central side ends (hereinafter, referred to as xe2x80x9cdistal endsxe2x80x9d) of the serration teeth on the outer circumferential surface of the yoke need to advance by partially cutting-out the inner circumferential surface of the shaft main body as if a groove is carved out with a chisel. In addition, a friction force generated at contact portions between the serration teeth and the inner circumferential surface of the shaft main body need to be overcome. Due to this, in the event that an axial compression force is applied to the yoke in a traffic accident or the like, unless the load so applied is greater than the sum of the cutting force and the friction force, the yoke is not allowed to sink into the shaft main body.
On the contrary, in the propeller shaft according to the first aspect of the invention, the extending portions of the serration fit-in grooves are formed in advance in the inner circumferential surface of the shaft main body before the yoke sinks thereinto. Due to this, when the yoke sinks into the shaft main body, almost no aforesaid cutting force which is required when the serration teeth advance cutting out partially the inner circumferential surface of the shaft main body is required, and in the event that an axial compression force of certain magnitude is applied to the yoke in a traffic accident or the like, the yoke is allowed to sink into the shaft main body with a load of small magnitude. To make it simple, when compared with the conventionally used propeller shafts, the sinking force needed to allow the yoke to sink into the shaft main body becomes smaller by such an extent that no inner circumferential surface needs to be cut out.
Thus, according to the first aspect of the invention, there is provided the propeller shaft in which, in the event that an axial compression load is applied to the yoke, the yoke is allowed to sink into the shaft main body with a force smaller in magnitude than that required with the conventionally used propeller shafts, thereby making it possible to improve the safety of an automotive vehicle incorporating the same propeller shaft of the invention.
Additionally, according to a second aspect of the invention, there is provided a propeller shaft, wherein the extending portions of the serration fit-in grooves are formed deeper than the serration fit-in portions. That is, in the propeller shaft according to this aspect of the invention, the groove depth of the extending portions where the serration teeth travel when the yoke sinks into the shaft main body is made deeper than that of the serration fit-in portions. Here, that the extending portions are xe2x80x9cformed deeperxe2x80x9d than the serration fit-in portions means that the shortest distance from the axial center line of the shaft main body to the bottom of the groove of the extending portion, that is to say, the radius, is greater than the radius from the same center line to the bottom of the groove of the serration fit-in portion.
In the propeller shaft according to the second aspect of the invention, since the grooves of the extending portions are formed deeper than those of the serration fit-in portions and the tightening force of the shaft main body is smaller, there is generated less friction force, and the serration teeth can travel smoothly when the yoke sinks into the shaft main body. Namely, the propeller shaft according to the second aspect of the invention allows the yoke to sink into the shaft main body with a smaller sinking force.
Furthermore, according to a third aspect of the invention, there is provided a propeller shaft, wherein the thickness of the shaft main body is made thicker within a predetermined portion ranging from the one end of the shaft main body where the yoke is mounted toward the axial center thereof, and wherein the axially central side ends of the extending portions of the serration fit-in grooves are located at positions coinciding with an axially central side end of the predetermined portion or closer to the one end of the shaft main body than the axially central side end of the predetermined portion.
Namely, the propeller shaft according to the third aspect of the invention has the portion which is made thicker at the one end portion of the shaft main body, and the serration fit-in grooves are formed on the inner circumferential surface of the thicker portion. In general, an end portion of a shaft main body made of FRP is formed thicker for reinforcement against the press fitting of a yoke thereinto by means of laminating an extra layer or layers of reinforced fibers. On the other hand, forming grooves on the inner circumferential surface of the FRP shaft main body reduces the torsional moment when the shaft main body fails from torsional buckling, that is, the torsional strength, and therefore tends to easily cause the torsional failure of the shaft main body. In the propeller shaft according to the third aspect of the invention, since the grooves are formed on the inner circumferential surface of the thicker portion or reinforced portion of the shaft main body and no groove is formed on the inner circumferential surfaces of any other portions than the reinforced portion, the reduction in torsional strength due to grooves can be prevented as much as possible.
There exists no particular limitation to methods for producing the aforesaid propeller shaft according to the invention, and therefore, the propeller shaft of the invention may be produced by using various conventional methods that have been used for producing propeller shafts. However, according to a propeller shaft producing method of the invention, which will be described below, the propeller shaft according to the invention may be produced easily with respect to joining a yoke to a shaft main body. According to a fourth aspect of the invention, there is provided a method for producing the aforesaid propeller shaft according to the invention comprising a yoke press fitting step of press fitting the yoke into the one end of shaft main body to a position where the teeth of the serrations fit in the extending portions while continuously forming the serration fit-in portions and the extending portions of the serration fit-in grooves on the inner circumferential surface of the shaft main body with the teeth of the serrations, and a yoke withdrawing step of withdrawing the yoke to a position where the teeth of the serrations fit in the serration fit-in portions.
That is, in the method for producing the propeller shaft according to the fourth aspect of the invention, in the yoke press fitting step, the serration fit-in grooves having the serration fit-in portions and the extending portions are formed on the inner circumferential surfaces of the shaft main body with the cutting force and the friction force of the serration teeth on the outer circumferential surface of the yoke, and in the yoke withdrawing step, the yoke is withdrawn to the position where the serration teeth fit in the serration fit-in portions of the serration fit-in grooves or a predetermined position where the yoke is put in operation in the normal operating condition. Speaking more simply, the grooves that mesh engage the serration teeth are formed by inserting the yoke to a deeper position in the shaft main body once, and thereafter, the yoke is withdrawn to the predetermined position where the yoke is put in operation in the normal operating condition. As a result of this, the extending portions are formed as spaces on a distal end side of the serrations.
In this production method, the serrations fit-in grooves are formed mainly in the yoke press fitting step. Described below will be the details of the formation of the serration fit-in grooves in the yoke press fitting step. First, the leading ends of the serration teeth formed in the outer circumferential surface of the inserting portion of the yoke cut out partially the relevant portions of the inner circumferential surface of the shaft main body as if a groove is carved with a chisel to thereby form notches. As the serration teeth travel along deeper inside the shaft main body, the notches so formed extend toward the axial center of the shaft main body to thereby form continuous grooves. At the same time, the grooves so formed by the leading ends of the serration teeth are made to become deeper when the grooves are brought into friction contact with the top and sides of the respective serrations each having a certain length in the axial direction as the yoke is press fitted into the shaft main body.
In the production method according to the fourth aspect of the invention, terminating points where the serration teeth which travel as the yoke is press fitted into the shaft main body stop moving in the yoke press fitting step or distal end positions of the serration teeth resulting at the end of the yoke press fitting step constitute the axial central side ends of the extending portions of the serration fit-in grooves. On the other hand, in the yoke withdrawing step, the yoke is caused to travel back along the serration fit-in grooves to the position where the serration teeth fit in the serration fit-in portions of the serration fit-in grooves. Consequently, terminating points where the distal ends of the serration teeth which travel back as the yoke is withdrawn stop moving constitute the axial central side ends of the serration fit-in portions of the serration fit-in grooves or axial end portion side ends of the extending portions, and terminating points where axial end portion side ends (hereinafter, referred to as xe2x80x9cproximal endsxe2x80x9d) of the serration teeth stop moving constitute axial end portion side ends of the serration fit-in portion of the serration fit-in grooves.
According to the propeller shaft production method of this aspect of the invention, the shaft main body and the yoke of the propeller shaft according to the invention can be joined to each other by performing extremely simple operations such as press fitting and withdrawing the yoke into and from the shaft main body. Even when compared with the conventional propeller shafts in which the shaft main body and the yoke thereof are joined to each other by performing a press fitting operation only, the production method of the invention simply adds operations of extra further press fitting the yoke and withdrawing the same yoke, and the method not only causes no remarkable loss in production efficiency but also requires no special devices. Consequently, with the production method according to the fourth aspect of the invention, the propeller shaft of the invention which only needs a sinking force of small magnitude while securing good rotating torque transmission properties can be produced in an extremely easy manner.
Additionally, in the propeller shaft according to the invention, in order to assure the proper torque transmission between the yoke and the shaft main body, the yoke is press fitted into the shaft main body the outside diameter (a diameter of a circle imaginarily formed by connecting top land portions of the respective serration teeth) of whose inserting portion is greater than the inside diameter of the shaft main body. Consequently, in the yoke press fitting step of the production method according to the invention, when the yoke is press fitted into one end portion of the shaft main body, a portion of the inner circumferential surface of the shaft main body through which the inserting portion of the yoke is about to pass is expanded in diameter within an elastic range of the shaft main body. Then, at the proximal end of the shaft main body through which the inserting portion of the yoke has passed, the shaft main body contracts in diameter by virtue of the elastic force (restoring force) of the shaft main body, whereby the diameter thereof is restored to the original diameter. Also in the yoke withdrawing step, the inner circumferential surface of the shaft main body similarly expands and contracts in diameter as the inserting portion of the yoke passes therethrough.
In view of these phenomena, the formation of the serration fit-in grooves is also performed by the serration teeth in the yoke withdrawing step as well. That is, there is generated another opportunity for the serration fit-in grooves which are once formed by the inserting portion of the yoke which passes therethrough in the yoke press fitting step to be formed deeper with the serration teeth as a result of the contraction of the diameter of the shaft main body. Then, also in the yoke withdrawing step, as with the yoke press fitting step, the proximal ends of the serration teeth in the inserting portion of the yoke act if they were a chisel to thereby carve further deeply the serration fit-in grooves formed in the yoke press fitting step.
Namely, the serration fit-in grooves formed through the additional yoke withdrawing step become deeper in groove depth when compared with the serration fit-in grooves formed only through the yoke press fitting step, and therefore, the fitting of the serration teeth into the serration fit-in portions of the serration fit-in grooves becomes stronger, thereby making it possible to assure further the proper transmission of rotating torque between the relevant components. Consequently, with the production method according to the fourth aspect of the invention, it is possible to obtain an advantage that the rotating torque transmission properties of the propeller shaft is improved.
With the aforesaid function of the production method being used in which the serration fit-in grooves are formed through both the yoke press fitting step and the yoke withdrawing step, according to a fifth aspect of the invention, there is provided a method for producing the propeller shaft as set forth in the fourth aspect of the invention, wherein a yoke reciprocating step is further included which is adapted to be performed at least once between the yoke press fitting step and the yoke withdrawing step and in which the yoke is withdrawn and press fitted such that the serration teeth at least reciprocate between a position where the serration teeth fit in the extending portions of the serration fit-in grooves and a position where the serration teeth fit in the serration fit-in portions of the same grooves.
That is, in this production method according to the fifth aspect of the invention, the inserting portion of the yoke is press fitted and withdrawn a plurality of times between a position where the yoke exists at the end of the yoke press fitting step (the xe2x80x9cposition where the serration teeth fit in the extending portionxe2x80x9d means this position) and a position where the yoke exists when the serration teeth are located in the serration fit-in portions of the serration fit-in grooves.
According to the production method of the fifth aspect of the invention, the serration fit-in portions and the extending portions of the serration fit-in grooves can be made deeper by repeating the yoke press fitting step and the yoke withdrawing step a plurality of times. With the deeper serration fit-in portions, as has been described above, the proper transmission of rotating torque between the yoke and the shaft main body is further assured. In addition, with the deeper extending portions, a friction force becomes smaller in magnitude which is generated between the serration teeth of the yoke and the extending portions when the yoke is caused to sink into the shaft main body by virtue of a certain axial compression load. Consequently, with the production method of the fifth aspect of the invention, the propeller shaft can be produced in an easy manner which can improve the rotating torque transmission properties and needs only a sinking force of small magnitude.
Additionally, according to a sixth aspect of the invention, there is provided a method for the propeller shaft, wherein the diameter of a portion where the extending portions of the serration fit-in grooves are formed in the inner circumferential surface of the shaft main body is contracted by applying a stress of certain magnitude to an outer circumferential surface of the shaft main body, and after the extending portions are formed, the diameter of the portion of the inner circumferential surface of the shaft main body is expanded by removing the stress therefrom, whereby the extending portions of the serration fit-in grooves are formed deeper than the serration fit-in portions of the same grooves.
In other words, in the propeller shaft production method according to the sixth aspect of the invention, the stress is applied from the outside to the portion of the shaft main body where the extending portions are formed so that the inside diameter of the portion is contracted, and in this state the yoke press fitting and withdrawing operations or either of the operations is performed so as to form the extending portions of the serration fit-in grooves. Then, after the extending portions are formed, the applied stress is removed, so that the diameter of the portion is expanded by virtue of the spring-back of the shaft main body. As a result, the extending portions of the serration fit-in grooves can be formed deeper than the serration fit-in portions of the same grooves. According to the propeller shaft production method of the sixth aspect of the invention, the propeller shaft can be produced in an extremely easy manner which can make a sinking force required by the yoke extremely low when sinking into the shaft main body by forming the extending portions of the serration fit-in grooves deeper than the serration fit-in portions of the same grooves.
Note that, as has been described heretofore, the shaft main body is expanded in diameter as the inserting portion of the yoke passes therethrough. Even if the portion where the extending portions are formed is not actually expanded in diameter, if the expansion of the shaft main body resulting from the passage of the inserting portion of the yoke is suppressed by a certain means, an effect can be obtained which corresponds to the effect provided by actually contracting the relevant portion. That is, even if no stress is actually applied to the outer circumferential surface of the shaft main body, if the outer circumferential surface of the relevant portion is restricted with a retainer band or the like, there is generated a reaction force when the inserting portion of the yoke passes through the restricted portion, this creating a condition where the stress is actually applied to the outer circumferential surface, whereby the diametrical expansion of the inner circumferential surface of the relevant portion is restricted. Consequently, the xe2x80x9ccontraction in diameterxe2x80x9d of the inner circumferential surface of the shaft main body at the portion where the expanding portions according to the aspect of the invention are formed means a relative contraction in diameter, that is, the restriction of the diametrical expansion of the relevant portion of the shaft main body that would be otherwise caused as the inserting portion of the yoke passes through the relevant portion. In addition, similarly, the xe2x80x9cexpansionxe2x80x9d means not only the actual expansion but also the restoration to the original inside diameter.